There are, in general, two types of fingerprint sensors, swipe/sweep sensors and area/placement sensor. Swipe sensors can be manufactured at lower cost than placement sensors as the sensing area of a swipe sensor is smaller than that of a placement sensor. Fewer interconnects, smaller sensor substrates, smaller ASIC (application-specific integrated circuit), etc. contribute to the lower cost of swipe sensors relative to placement sensors. Swipe sensors, however, are considered less user-friendly than placement sensors. This is mainly due to the fact that some training of the user on how to swipe the finger across the swipe sensor is necessary. For a placement sensor, on the other hand, the finger is simply laid over, or placed on, the sensor, which might be more intuitive to the user. The downside of placement sensors is the need for more complex electronics due to the many more sensing elements, leading to larger ASICS and more complex interconnects, and hence higher cost.
European Patent Publication EP1353294 and U.S. Pat. No. 7,190,816 describe a system utilizing a sweep fingerprint sensor where the finger is registered in the system in the normal way, by making a full finger swipe. The swipe sensor can thereafter be used in a strict authentication mode and an easy authentication mode. In the strict authentication mode the finger needs to perform a full swipe to fulfill the authentication, while in the easy authentication mode the finger might lay still on the swipe sensor and an image of only a small part of the fingerprint is generated. The small image is used to authenticate the user with a lower security level, suitable for releasing a dial lock or other low-security level activity.
U.S. Pat. No. 7,369,684 describes a system using multiple sweep sensors in which each sweep sensor has multiple linear sensing arrays. The linear sensing arrays of the first sweep sensor have a first regular spacing, the linear sensing arrays of the second sweep sensor have second regular spacing, which may be identical to the first regular spacing, and the multiple sweep sensors are spaced apart such that the spacing between them is other than an integer multiple of the first known spacing. The idea of this patent is to use multiple sweep sensor to increase the resolution of the image, hence the importance of the non-integer multiple of the spacing between them. Each sweep sensor of this application is a multi-line sweep sensor, as would be required for image stitching as described therein.
U.S. Pat. No. 7,734,074 describes using a plurality of linear finger sensing arrays in order to generate a respective plurality of finger image data sets, determining the movement of the finger based upon the finger image set, generating a resampled finger image data set based on the finger movement by arranging the plurality of finger image data sets into a single array to generate a supersampled finger image data array having a resolution greater than a resolution of each finger image data set, and then downsampling the supersampled finger image data array to generate a downsampled finger image data array having a resolution lower than the resolution of the supersampled finger image data array and thereby defining the resampled finger image data set. The patent states that this is in contrast to approaches that use various types of finger speed sensors to control the sampling rate or discard repeated data, such as disclosed, for example, in U.S. Pat. No. 6,002,815, or use cross-correlation between partially overlapped image frames to eliminate redundant data and stitch the data together into a single global image, such as disclosed in U.S. Pat. No. 6,289,114.
Based on the discussion above a benefit may be achieved by providing a sensor system that combines the low cost of the swipe sensor system with the user friendliness of the placement sensor. This is obtained as stated in the accompanying claims.
The invention will be described more in detail below with reference to the accompanying drawings, illustrating the invention by way of examples.